The expression of μ-opioid receptors
in the developing rat spinal cord (Postnatal days 7, 14, 30) was studied by
autoradiography using [3H]DAMGO. When compared to camera lucida
drawings, the receptor was noted over the entire gray matter and dorsal root
ganglia at postnatal days 7 and 14. At postnatal day 30, the receptor
expression decreased over the gray matter except the superficial laminae
(laminae I and II). At all age groups studied, a higher expression of the receptor
was noted over the superficial laminae. The study shows that μ-opioid receptors
appears early in postnatal development and attains mature receptor distribution
relatively late in ontogeny, suggesting a possible role in the normal development
of nervous system. This is affirmed by an impairment of psychomotor development
in babies born to mothers, addicted to opiates.